The present invention relates to a method for repairing a porous refractory material and, more particularly, to a method of repairing surface damage to a refractory material which is coated with a glass coating.
Portions of the space shuttle orbiter, which is subject to temperatures ranging from -250.degree. F. to 2300.degree. F., utilize tiles formed of substantially chemically pure silica fibers which are rigidized with a high purity silica binder. The tiles, known as L1-900, are manufactured by Lockheed Missles and Space Company, Inc., have a density of about 9 pounds per cubic foot, relatively low strength, extremely high temperature resistance, and an extremely low coefficient of thermal expansion as compared to metals. The tiles, because they are porous, have a tendency to act as a sponge in terms of absorbing water and, moreover, do not possess requisite optical properties either in terms of being optically absorbent or reflective depending on their point of attachment to the space shuttle orbiter. To waterproof the tiles, impart the necessary optical properties and facilitate handling of the tiles to avoid damage, the tiles are coated with a relatively thin glass like coating. The glass like coating, which is generally a borosilicate glass, is itself very susceptible to damage.
Surface damage to the tiles can cause serious problems when the space shuttle orbiter is in flight. Loss of the engine coating results in a loss of optical properties which interferes with proper heat emittance. Accordingly, the temperature may rise excessively, i.e. above 2300.degree. F., causing burn through of the tile to the metallic understructure and of course burn through of the understructure. Additionally, surface damage to the glass coating and the tile creates turbulent, non-streamline air flow resulting in cavitation in the damaged area with the result that the tile will be subjected to forces which can physically tear it apart. This can ultimately result not only in loss of the tile which has been damaged but may result in peeling off of adjacent tiles with a result that the metallic understructure of the orbiter would then be exposed to destructive temperatures. Since the individual tiles are contoured and fitted, it is not practical to simply replace the damaged tile with a new tile. Accordingly, it is necessary to have a method by which the damaged tile can be repaired and returned to its original integrity prior to flight.
U.S. Pat. No. 2,791,262 discloses a process for improving the physical characteristics of sheet form which includes the steps of forming a mixture of mica pulp and hydrolyzed ethyl silicate, forming a sheet from the above mixture and heating the formed sheet. U.S. Pat. No. 4,093,771 discloses the use of silicon tetraboride in glass coatings used on silica tiles. Neither of the above patents, nor U.S. Pat. No. 4,124,732 which discloses the system for attaching the ceramic tiles to the orbiter spacecraft, and which are the closest prior art known to applicants, teach a technique for effecting repair to a porous refractory substrate coated with a glass coating which restores the damaged tile to its original integrity.